Offset antenna configurations comprising a reflector with geometrically shaped surface (in English: offset shaped reflector antenna) and a primary source shifted with respect to the axis normal to the reflector, engender radiations in a cross-polarization induced by the geometric curvature of the reflector and the level of which depends directly on the focal ratio of the reflector, the focal ratio being defined by the ratio of the focal length to the diameter of the reflector. The larger the focal ratio, the lower the level of cross-polarization. However, when the antenna is fitted on an Earth-ward oriented face of a satellite, the structure of the antenna must be compact and the focal ratios are low, thereby inducing a high level of cross-polarization.
In the case of an antenna comprising a reflector illuminated by a centered primary source, the level of cross-polarization is zero in the direction normal to the antenna but there may be axisymmetric cross-polarization lobes due to the curvature of the field lines at the ends of the reflector.
Moreover, the primary source used may, when its performance is low, itself engender field components comprising a cross-polarization.
To meet specifications of low cross-polarization level, satellite-mounted Earth-ward pointing antennas often have a double-reflector structure mounted in a Gregorian configuration. The use of two reflectors makes it possible to define the geometry of the auxiliary reflector with respect to the geometry of the principal reflector in such a way that the cross-polarization induced by the curvature of the auxiliary reflector cancels the cross-polarization induced by the curvature of the principal reflector. However, the presence of the auxiliary reflector and of its support structure gives rise to an increase in the mass, volume and cost of the antenna with respect to an antenna with a single reflector.
Another solution for decreasing the cross-polarization level is to use a reflector array antenna (in English: reflectarray antenna) in an offset configuration. In this type of antenna, a primary source illuminates a reflector array at oblique incidence. The reflector comprises a set of elementary radiating elements assembled into a one- or two-dimensional array and forming a reflecting surface which may be plane. By considering the case where the radiating elements of the antenna are all identical and do not individually induce any cross-polarization, the reflector array then acts as a mirror and the radiation reflected by the reflector array does not comprise any cross-polarization component if it is illuminated by a primary source free of cross-polarization placed on its axis of symmetry. However, the radiating elements of a reflector array generally comprise geometric differences so as to precisely control the phase shift that each radiating element produces on an incident wave. Furthermore, the layout of the elementary radiating elements with respect to one another on the surface of the reflector is generally synthesized and optimized so as to obtain a given radiation diagram in a chosen direction of pointing with a chosen phase law. Consequently, it has been noted that although the reflector is plane and that there is therefore no cross-polarization induced by the curvature of the reflector, on account of the illumination of the reflector by a source in the offset configuration, the reflector array behaves in operation as a reflector with geometrically shaped surface which also induces a cross-polarization radiation whose level is of the same order of magnitude as an equivalent reflector with shaped surface.